Project Summary Recent human genetic studies identified a link between a class of centrosomal proteins and microcephaly, which is characterized by a selective reduction of brain size in comparison to other organs. The goal of this proposal is to establish a novel mechanism of microcephaly by understanding how dysregulation of mitotic progression and cell cycle re-entry leads to neural progenitor cell (NPC) reduction in microcephaly. This contrasts with the dominant model in the field that disruption of symmetric/asymmetric division of NPCs causes microcephaly. We will test this mechanism by focusing on WDR62 (MCPH2; OMIM 604317), which is the second most common genetic cause of human microcephaly and encodes a WD-40 repeat protein. We created a hypomorphic mouse model of Wdr62 deficiency and found that mutant mice exhibited reduced brain sizes due to a decrease in NPCs. Wdr62 deficient NPCs exhibit mitotic progression delay and an increase in cell death. Wdr62 deficient mouse embryonic fibroblasts (MEFs) showed reduced spindle stability and spindle assembly checkpoint (SAC) activation. Wdr62 physically and genetically interacts with Aurora A, an established spindle assembly factor. In addition, Wdr62 localizes to the basal bodies of primary cilia and regulates cilia disassembly and cell cycle re-entry of MEFs. Depletion of Cep170, another Wdr62 interacting protein, also results in cilia disassembly, suggesting that Wdr62 may function together with Cep170 to regulate cilia biogenesis and cell cycle progression. These preliminary data lead to a novel hypothesis that Wdr62 regulates neural progenitor expansion in the developing brain by influencing mitotic progression and cell cycle re-entry, which are disrupted by disease mutations in a specific manner. To test this hypothesis, three specific aims will be pursued: 1) Test the hypothesis that Wdr62 regulates mitotic progression of neural progenitor cells (NPCs) by influencing spindle integrity; 2) Test the hypothesis that Wdr62 regulates cilia disassembly and cell cycle re-entry by functioning together with Cep170; 3) Test the hypothesis that individual disease alleles of WDR62 compromise its specific functions (mitosis or cilia disassembly) due to loss of regulation of specific Wdr62 interacting proteins. Together, these studies will improve our understanding of mitosis and cell cycle re-entry regulation of NPCs in the developing brain and provide novel insights into mechanisms underlying human microcephaly diseases.